1. Field of the Invention
The invention relates to aqueous silicone emulsions which comprise high-viscosity polyorganosiloxanes and have a particularly low content of cyclic siloxanes, to methods for the production thereof, and to the use thereof.
2. Description of the Related Art
Silicones can be used widely. In order to facilitate the use and dosing, especially in the case of viscous products, it is desirable for many applications that the organosilicon compounds are present in dilute form. Although the use of organic solvents, such as benzene or chlorocarbons, is possible for this purpose, it is disadvantageous from an ecological and occupational medicine point of view. Consequently, they are used mostly in the form of aqueous emulsions or dispersions, usually as oil-in-water emulsions (O/W emulsions), which can be diluted with water. The oil phase here is understood as meaning the water-immiscible organosilicon compounds, optionally dissolved in organic solvents.
For many applications, it is advantageous if the silicone has a high molecular weight and thus a high viscosity. A known way to arrive at emulsions which comprise a high molecular weight silicone is the emulsion polymerization of low-molecular weight, in particular cyclic, organosiloxanes with arylalkylsulfonic acids (DE-B 14 95 512). In this process, as a result of intensive stirring or homogenization using a high-pressure homogenizer, extraordinarily low particle sizes are achieved which can no longer be seen using an optical microscope. A disadvantage of this method is the fact that, on account of the equilibrium character of this reaction, based on siloxane, more than 10% volatile cyclic siloxanes are present, but these are undesired. Consequently, it has been proposed to subsequently distill these off (e.g. U.S. Pat. No. 4,600,436) or to remove them using a membrane method (EP-A 1 368 109). Both methods signify additional technical expenditure and can adversely affect the stability of the emulsion.
Alternatively, instead of cyclic siloxanes, linear oligomers with terminal silanol groups can be used. From these oligomers, in the presence of emulsifiers, condensation catalysts and a very small amount of water, a paste is formed in which the polycondensation takes place. This paste is then diluted to the desired concentration (EP 93 310 B2). In general, the fractions of cyclic volatile siloxanes are lower than in the case of the emulsion polymerization of cyclic siloxanes. A reduction in the fraction of these volatile siloxanes can take place e.g. by firstly producing an emulsion from the salt form of the anionic emulsifier/catalyst, and then activating this by adding acid (EP-A 1 072 629). This ultimately increases the salt fraction in the emulsion, which is disadvantageous for the stability. When using alkoxy-terminated siloxane oligomers, likewise fewer cycles are said to be formed (JP-A2001288269). However, these oligomers are more difficult to produce and thus more cost-intensive.
Special emulsifiers based on taurocholates likewise contribute to the reduction in the amount of cycles which are formed during the emulsion condensation of siloxane oligomers (WO 2006102010). However, here too, as the working examples clearly show, more than 1% octamethylcyclotetrasiloxane is formed.
It has also been proposed to emulsify dimethylpolysiloxanes, in particular polysiloxanes which are terminated with trimethylsiloxy groups, with viscosities of up to 5,000,000 cSt by mixing and heating these with 10-30%, based on siloxane, of a phosphoric acid partial ester until a clear solution is formed which, after neutralization, is diluted with water (DE-A 27 30 923). However, this method has the disadvantage that the polydimethylsiloxane is in most cases depolymerized, meaning that the resulting emulsion comprises a low-viscosity siloxane and a high fraction of volatile cyclic siloxanes, e.g. octamethyl-cyclotetrasiloxane.
In JP2002020490 it is proposed to use a two-component combination of emulsifiers comprising polyoxyethylene alkyl sulfates, polyoxyethylene alkyl phosphates and alkylsulfonates or the corresponding acids, in which case the acid is preferably only liberated in the emulsion by adding mineral acids such as sulfuric acid. The sole use of polyoxyethylene alkyl phosphates is said to lead only to low molecular weight polyorgano-siloxanes since their catalytic activity is too low. Consequently, combinations with sulfates or sulfonates and an activation by sulfuric acid are necessary. This leads ultimately in turn to more than 1% cyclic siloxane oligomers unless the reaction time is extremely short, although in this case no viscosities >1,000,000 mm2/s are achieved.
On the other hand, emulsions of this type are in practice often produced by either producing a plurality of batches discontinuously and transferring them to a ripening tank or producing a continuous campaign over a certain period in a ripening tank where then, after reaching the desired viscosity, the reaction is terminated by neutralization. In this connection, it is inevitable that a not inconsiderable part of the emulsion remains in the tank for longer than necessary, as a result of which the fraction of cyclic oligomers exceeds the tolerable level.